Quitting smoking can lead on average to 4kg to 5kg in weight gain within a year but experts say the benefits far outweigh the extra kilos.

A large international study published this week in the British Medical Journal analysed 62 clinical trials of people who sought medical help to stop smoking. After 12 months of abstinence, people gained on average 4kg to 5kg. However, 16 to 21 per cent of people actually lost weight.

A larger portion, 35 to 38 per cent, gained less than 5kg, 29 to 34 per cent gained 5kg to 10kg, and 13 to 14 per cent put on more than 10kg.

However, the wide variation in weight gain mean the results cannot be applied to everyone who quits smoking, the study's authors said.

Simon Chapman from the Sydney School of Public Health said the results were concerning because they could deter people indefinitely from quitting.

Professor Chapman said the data was unreliable because it was gleaned from clinical trials of people who sought help to stop smoking, not the general population.

Smokers who enrolled in trials differed from those who quit without help, he said.